Fix async and defer options when loading scripts

[dawid-in]

Options are stored as value in the array, rather than a key.

[x] I have read and followed the contribution guide: https://github.com/pagekit/pagekit/blob/develop/.github/CONTRIBUTING.md
[x] The destination branch of the pull request is the develop branch

[ahoiroman]

Hi @dawid-in

Could you please tell us a bit more about your pull request? I think it's still possible to use async and defer, using something like:

$app[ 'scripts' ]->add('foo', 'bar', $dependencies, ['defer' => true, 'async' => true]);